The main goal of the proposed project is to examine diverse intermolecular complexes involving nucleic acid bases by using the newly developed direct and semidirect ab initio methods for studying intermolecular interactions. Initial emphasis will be on binary complexes of nucleic acid bases, for which preliminary results are provided. To better understand the nature of multiple N-H-O=C H-bonds, calculations of interaction energies for the formamide and acetamide dimers will be performed, examining different tautomeric forms as well as some ionized forms of nucleic acid bases. Exploratory calculations of three-body interactions of H-bonded and stacked trimers of nucleic acid bases are also proposed and an extension of the present approach to the more sophisticated methods of treating electron correlation will be continued. From a practical point of view, a most important goal of this proposal is to develop accurate model potential energy surfaces. The availability of the component energies will permit an independent fitting of each kind of interaction separately and reduce the computational intensity of producing a potential surface. Further applications of such potentials will include modeling of ligand-DNA intercalation interactions.